DESCRIPTION: (Applicant's Abstract) Retinopathy of Prematurity (ROP) is a vasoproliferative disease affecting the retina of premature babies and is the major cause of blindness in children. ROP has been associated with low birth weight and oxygen supplementation, but its pathogenesis remains controversial. Much attention has focused on the possible role of the vascular endothelial growth factor, VEGF-A, in the development of this disease. However, existing models of ROP do not allow its proposed role in the disease process to be tested easily. We have established an in vitro system in which to investigate the role of VEGF-A and other vascular factors during normal retinal vascularization and ROP pathogenesis. In this model, retinal explants from newborn mice were maintained under in vitro conditions, which allowed the immature vasculature to develop and differentiate in a manner similar to that observed in vivo. Exposure of retinal explants to hyperoxia produced a vascular response similar to that obtained with the in vivo murine model for ROP. Our model allows easy manipulation of VEGF availability by addition of VEGF or anti-VEGF antibodies to the culture medium. In preliminary results, addition of neutralizing antibodies to the retinal or-an culture caused a severe regression of blood vessels throughout the whole retina that did not accurately mimic the vessel regression that characterizes the initial phase of ROP. Addition of VEGF-A into the culture medium of retinal explants during oxygen exposure inhibited the upregulation of the pro-apoptotic protein Bax, supporting a protective role of VEGF-A from oxygen-induced apoptosis. Together, these results suggest that alterations in VEGF-A availability play a role in the response of the retinal vasculature to hyperoxia, but other factors are also involved. We propose to further characterize our explant model by examining expression of the different VEGF-A isoforms, other members of the VEGF family, and the angiopoietins. We will determine if the expression pattern that occurs in retinal vascular development and in the response of the developing vessels to hyperoxia in vivo is accurately reflected in our model. In addition, we will use our model to test the specific roles of VEGFs and angiopoietins in developing normal or hyperoxic vascular bed by addition of the growth factors or their inhibitors to the retinal explant cultures. Our model will provide a useful tool to directly examine the roles of various factors in retinal vascular diseases in a setting that maintains normal tissue architecture.